Power devices that are low loss, compact, highly functional devices capable of operating with high current and high voltage are most promising as next-generation semiconductors. As development of such devices continues to advance in recent years, the requirements for sealing materials for Si, SiC or GaN devices have become ever more stringent. In particular, there is high demand for use in high-temperature environments, i.e., greater heat resistance, to allow driving at higher power. Relatively high heat-resistant polyimide resins, silicone gels and high heat-resistant epoxy resins have therefore been used in the prior art as sealing materials for power devices.
Polyimide resins (with glass transition temperatures of 350° C. or higher) have high heat resistance but also poor workability, requiring high temperatures and long time periods for molding. Silicone gels (with glass transition temperatures of 400° C. or higher, or else not observed) are used in potting-type molding and therefore require cases that can support their shapes during molding, and the resins themselves are expensive, rendering them disadvantageous in terms of cost and productivity. Heat-resistant epoxy resins (glass transition temperatures of 100 to 200° C.) have excellent workability but are inferior to the aforementioned two types of materials in terms of heat resistance, including high-temperature mechanical properties and electrical characteristics. Moreover, since heat-resistant epoxy resins have special structures with a naphthalene backbone or tetraphenyl backbone, they are costly and limited in their practicality.
On the other hand, thermosetting resins with high heat resistance are known, namely compositions including an alkenylphenol compound and an aromatic bismaleimide compound (for example, see Patent Document 1 (Japanese Unexamined Patent Publication No. H5-43630) and Patent Document 2 (Japanese Unexamined Patent Publication No. H6-93047)). A cured resin with high heat resistance can be obtained by radical polymerization between the alkenyl groups of the alkenylphenol compound and the unsaturated groups of the aromatic bismaleimide compound, to produce a high degree of crosslinking. Such a thermosetting resin (having a glass transition temperature of 200 to 350° C.) has heat resistance that is inferior to that of a polyimide resin or silicone gel, but still exhibits higher heat resistance than a heat-resistant epoxy resin while also allowing transfer molding similar to epoxy resins, and therefore such resins are known to exhibit both heat resistance and molding workability.
However, since compositions including alkenylphenol compounds and aromatic bismaleimide compounds have higher crosslinking point density than epoxy resins, the impact resistance of their cured products are low due to a high flexural modulus, and therefore when cured they are hard and brittle. Moreover, when the compositions are used as sealing resins, because of the phenolic hydroxyl group in the alkenylphenol compound that remains without contributing to polymerization, the high-temperature electrical characteristics, the heat degradation resistance and the humidity resistance of the sealed apparatus may be impaired.
In order to solve these problems, Patent Document 3 (Japanese Unexamined Patent Publication No. S62-280254) discloses a thermosetting resin composition comprising an allyl etherified substituted phenolic novolak resin wherein the phenolic hydroxyl group in an alkenyl (allyl) phenol has been alkenyl (allyl) etherified, a N,N′-bismaleimide compound, and an epoxy resin.
However, the aforementioned thermosetting resin composition cannot provide a cured product with high heat resistance because it includes an epoxy resin. Furthermore, Claisen rearrangement reaction proceeds under high-temperature conditions in alkenyl ether compounds. This results in problems such as low long-term heat resistance of the cured product due to the chemical instability of the allyl ether groups remaining after curing of the composition, and a large degree of cure shrinkage.